In many applications, it is desired to acquire data from numerous distributed sensors. For example, in a seismic data acquisition system, it may be desirable to acquire data from numerous distributed accelerometers or other seismic sensors. In order to accurately compare and process data collected from the various sensors, it is often necessary or advantageous that data samples collected from the various sensors are synchronized to a universal time. For example, in U.S. Pat. No. 7,548,600, the data acquisition system disclosed thereon time stamps samples taken at the clock rate of a crystal oscillator local to the sensor to a reference time derived from an accurate clock measurement (e.g., from a satellite-based positioning system). However, these and similar techniques require that a reference filter for performing the time stamping be calculated “on the fly,” for example by interpolation of a highly over-sampled table. Thus, for 24-bit resolution using such techniques, at least 12 bits of oversampling (i.e., a factor of 4096) are needed for such accuracy if linear interpolation is assumed. Accordingly, the reference filter utilizes significant memory, and the size of such memory and the power necessary to retrieve and calculate the reference filter may be disadvantageous. For example, U.S. Pat. No. 8,260,580 notes expense, high computing load, a strong memory constraint, and a high consumption in energy when it describes these or similar techniques. In addition, existing seismic systems may use temperature-controlled crystal oscillators, which may have a high cost and require significant power. Components and techniques that require significant amounts of power may be disadvantageous for use in distributed sensor systems in which sensors are typically battery powered.
The use of a temperature-controlled crystal oscillator may also increase system complexity by requiring a feedback loop programmed in a microcontroller unit. The loop requires synchronization to an accurate clock (e.g., from a satellite-based positioning system), measuring temperature, and using a digital-to-analog converter to control the control voltage of the temperature-controlled crystal oscillator. The microcontroller unit closes the control loop for this virtual phase-locked loop, which further complicates the systems and requires more power.
In addition, correction circuitry of a temperature-controlled crystal oscillator is a significant source of measurement error in existing approaches, and “pulling” of a crystal oscillator in a temperature-controlled crystal oscillator also leads to instability of the oscillator.